1. Field of the Invention
The present invention relates to systems and techniques for determining calibration parameters for a lithographic process.
2. Related Art
Photo-lithography is a widely used technology for producing integrated circuits. In this technique, a light source illuminates a photo-mask. The resulting spatially varying light pattern is projected onto a photoresist layer on a semiconductor wafer by an optical system (which is sometimes referred to as an exposure tool). By developing the 3-dimensional pattern produced in this photoresist layer, a layer in the integrated circuit is created. Furthermore, because there are often multiple layers in a typical integrated circuit, these operations may be repeated using several photo-masks to produce a product wafer.
As dimensions in integrated circuits steadily become a smaller fraction of the wavelength of the light used to expose images of the photo-mask onto the wafer, it is becoming increasingly difficult to design and manufacture photo-masks that produce the desired target wafer pattern on the wafer. As a consequence, the structures in or on the ideal photo-mask (also referred to as the target mask pattern) and/or the physical structures in or on the actual photo-mask bear less and less resemblance to the desired target wafer pattern. These differences between the photo-mask and the target wafer pattern are used to compensate for the diffraction and optical-proximity effects that occur when light is transmitted through the optics of the exposure tool and is converted into the 3-dimensional pattern in the photoresist.
When designing a photo-lithographic process (including the target mask pattern(s) for the photo-mask(s) and/or the corresponding source pattern(s)), the manufacturing performance of a given solution is often evaluated using photo-lithography simulations. In order to improve the accuracy of the predicted manufacturing performance, a lithographic simulator that performs these photo-lithography simulations is usually calibrated based on a test pattern that is produced using the photo-lithographic process. However, the calibration is often only performed for a limited range of lithographic conditions in the photo-lithographic process. Consequently, the simulations may not be accurate over the full range of lithographic conditions in the photo-lithographic process, which may adversely impact the predicted manufacturing performance.
Hence, what is needed is a calibration technique that overcomes the problems listed above.